270 
F. Cavers. 
The vast majority of the genera and species of Hepaticae belong 
to this great group, which lias a world-wide distribution but is 
especially developed in the wet forest regions of the tropics. In 
round numbers, the Acrogynae include all save some 60 genera and 
700 species of the 250 genera and 4,500 species of Hepaticae which 
have been described up to the present time. In Britain, there are 
only some 70 genera and 250 species of Hepaticae, and of these 50 
genera and 200 species belong to the Acrogynae. These figures 
show the striking preponderance of the Acrogynae over the rest of 
the Hepaticae ; and the poverty in species of the Hepatic flora of 
Britain, which resembles the North Temperate regions generally in 
this respect, is shown by the fact that there are several Acrogynous 
genera with more species than the total number of British Hepaticae, 
e.g. Plagiochila with over 500 species, Frullania with about 350, 
Bazzama with upwards of 300. 
Before dealing with the classification and inter-relationships of 
the Acrogynae, we may consider the chief morphological characters 
of the group, with special reference to the wealth of parallel develop¬ 
ments which is one of its most striking features. 
The size of the plant varies considerably in the Acrogynae. 
Some of the epiphytic Lejeuneas, which grow on living leaves of 
ferns and flowering-plants, are only a few millimetres long and 
consist of delicate thin-walled cells, while some of the large bark- 
inhabiting forms of the same group (e.g. Bryopteris) reach a length of 
several feet and are firm and hard in texture. In his travels on the 
Amazon, Spruce found remarkable pendent festoons of epiphytic 
Frullanias, which often formed an important feature in the vegetation 
of wet tropical forests and which were too thick for a man’s arms 
to surround them. 
In the Acroygnae, the apical cell of the shoot (Fig. 44) has 
almost invariably the form of a three-sided pyramid, from which 
three sets of segments are cut off. One side of the apical cell is 
parallel with the ventral surface of the stem, and the other two 
sides meet in the middle line above, hence one set of segments is 
ventral and the other two sets dorso-lateral. From each of the 
latter a leaf is formed, while the ventral segment may or may not 
produce an underleaf (amphigastrium). The form of the apical 
cell is directly related to the size of the underleaf as compared with 
the lateral leaves. Where the underleaves are well-developed, the 
three sets of segments are about equal in size, and the apical cell 
forms in surface view an equilateral triangle; where no underleaves 
